1. Field of Invention
The invention described below generally relates to thermoplastic vulcanizate compositions (TPVs), preferably “soft” TPVs, i.e., those having low hardness levels.
2. Description of Related Art
Thermoplastic vulcanizate compositions (TPVs) typically include a rubber component, a thermoplastic component, additive oil and other ingredients such as curing additives and the like. Specific examples of TPVs are seen in U.S. Pat. No. 6,288,171. A variety of properties of TPVs are considered to be important, including but not limited to rebound, tensile strength and elongation, as well as varying degrees of softness or hardness. One of the many challenges in making TPVs is to obtain the right balance of properties.
Thermoplastic elastomer blends having a cured rubber content are well known in the art. See, e.g., ASTM D 1566-03. In U.S. Pat. No. 4,130,535, a cured (vulcanized) thermoplastic elastomer, also referred to as a thermoplastic vulcanizate or TPV, is made from a polyolefin resin and an olefin rubber. The at least partially cured state of the thermoplastic elastomer is obtainable by subjecting the blend of uncured rubber and thermoplastic resin to a curing process. This process can be static or dynamic and be done by the use of known curing agents, like peroxides or phenolic resins.
A dynamically vulcanized polyolefinic thermoplastic elastomer typically consists of 1-5 micron sized crosslinked rubber particles (typically the major phase) in a continuous semi-crystalline polyolefin matrix (typically the minor phase). TPV composition processability and physical properties (such as tensile strength) within the elastomer service temperature range are due at least in part to the continuous semi-crystalline polyolefin matrix. The matrix is the “hard” phase as opposed to the rubber “soft” phase (where the terms “hard” and “soft” are understood as being relative to one another). The hardness of the TPV composition depends on the relative amounts of soft and hard phases present. Reducing the “hard” semi-crystalline polyolefin phase would increase TPV softness at the expense of decreased product processability. A lower amount of the matrix, or, if the matrix is insufficient to provide a continuous phase, can result in TPV rubber particle agglomeration which would manifest itself as a “powdery” instead of a “continuous” thermoplastic on processing, for example, by melt extrusion of the product in compounding or finishing.
Physical properties also tend to be lowered when an insufficient matrix “glue” is present that holds the crosslinked rubber particles together. Thus, desirable soft TPV compositions are not necessarily readily produced. Moreover, increased TPV rubber content alone will not afford soft TPV compositions; added additive oil is necessary. But TPV compositions containing excess oil often tends to swell the rubber phase and therefore reduce plastic phase volume, which may result in the already mentioned disadvantages. Also, low molecular weight rubber and/or plastic molecules that are unattached to the TPV network can render the product “sticky” in the presence of oil. Unattached plastic molecules are those not incorporated into the polyolefin crystallites and unattached rubber molecules are those that are not bound to the crosslinked rubber network. Thus, it is often difficult to commercially produce viable TPV compositions with the desired levels of softness (low hardness).
Crosslinking of the rubber phase, e.g., during vulcanization, tends to increase TPV hardness, but TPV compositions containing a large amount of uncrosslinked rubber do not necessarily provide commercially viable soft products. The ethylene content in many commercially available EPDM rubber generally varies from about 50 weight % to about 70 weight %. The incorporation of uncrosslinkable EP rubber into a TPV composition or the addition of EPDM rubber to a preformed TPV composition will not necessarily yield a suitable soft product. The uncrosslinked rubber would be compatible with the crosslinked rubber phase, and will tend to be included into this phase, and thus the previously mentioned drawbacks of a high rubber content TPV composition will not be overcome. If an excessive amount of uncrosslinked rubber is present, or if all the rubber in the polyolefinic thermoplastic elastomer is uncrosslinked, then this “soft” product would suffer the processing and property disadvantages of a continuous rubber phase as opposed to those of the desirable continuous plastic phase for the thermoplastic elastomer product.
Certain thermoplastic elastomers targeted to have a Shore A Hardness of less than 35 are described in EP-B-0 892 831. That patent describes a blend of rubber and thermoplastic resin, the rubber being at least partially cured, comprising a) the thermoplastic resin, b) an uncured amorphous poly-α-olefin or an ethylene and C3-20 α-olefin amorphous copolymer and c) an at least partially cured rubber, wherein the weight ratio of b) to a) plus b) is 15-75 wt. %, and where weight ratio of c) to a) plus b) plus c) is 25-75 wt. %. Additionally, “soft polyolefins” are addressed in the article “Versatile New Soft Polyolefin For Compounding With Other Soft Thermoplastics Resins or as a TPV Base Resin”, L. Struzik, et al. (ANTEC 2003). This soft polyolefin is described as an in-situ polyolefin that has high rubber content prepared by selection of catalyst and process technology. The soft polyolefin is taught for use where elastomeric properties are sought by either blending with other polyolefins or by a process of dynamic vulcanization of the rubber content in the soft polyolefin. The examples illustrate compositions having Shore A hardness at or above about 60.